In a seawater desalination plant, a seawater desalination apparatus (hereinafter, referred to as a desalination apparatus) is used for obtaining fresh water from raw water (seawater) through desalination treatment so that clean water can be used.
Such a desalination apparatus is provided with a pretreatment device using a RO membrane (reverse osmosis membrane), a UF membrane (ultrafilter membrane), or a MF membrane (microfilter membrane) in order to remove suspended matters in seawater as raw water. Furthermore, chlorination for adding a chlorine agent (chlorine-containing water) to the raw water is generally carried out on the raw water in order to perform sterilization, eliminate algae, and remove organic matters, iron, manganese, or ammonia. In the chlorination, liquid chlorine, sodium hypochlorite, chlorine obtained by electrolysis of salt water, or the like is used.
Those obtained through the chlorination and filtration treatment are subjected to the desalination treatment by a reverse osmosis membrane device having a RO membrane.
Incidentally, although problems may not occur when the RO membrane is made of a material with chlorine-resistance (e.g., cellulose acetate), when a material without chlorine-resistance (e.g., polyamide membrane) is used as the RO membrane, it is necessary to neutralize chlorine with a reducing agent at the preceding stage of the reverse osmosis membrane device.
As the reducing agent, sodium bisulfate (referred to as “SBS”) for example is used (see Patent Literatures 1 to 3).
An example of a desalination apparatus that performs conventional chlorination and neutralization treatment is illustrated in FIG. 5.
As illustrated in FIG. 5, a conventional desalination apparatus 100 includes a pretreatment device 13 having a pretreatment membrane 13a that filters suspended matters in raw water 11 to which chlorine-containing water 12 is supplied, a reverse osmosis membrane device 17 having a reverse osmosis membrane (RO membrane) 16 that removes a salt content from filtrate water 14 supplied from the pretreatment device 13 to thereby produce permeated water 15, and a reducing agent tank 19 that supplies SBS solution 18 to the filtrate water 14 on an upstream side of the reverse osmosis membrane device 17. The desalination apparatus 100 removes the suspended matters and performs chlorination. In FIG. 5, the reference numeral 20 denotes concentrated water, 21a and 21b denote solution sending pumps, 22 denotes a raw water line, 23 denotes a filtrate water line, 24 denotes a concentrated water line, and 25 denotes a permeated water line.
Patent Literature 1: Japanese Patent Application Laid-open No. H7-308671
Patent Literature 2: Japanese Patent Application Laid-open No. H7-171565
Patent Literature 3: Japanese Patent Application Laid-open No. H9-57076